1. Field of the Invention
The present invention relates to a method for the correction of the uniformity of a gamma camera.
The invention can be applied to particular advantage in the field of medical imaging, implementing gamma camera type viewing devices.
A gamma camera generally comprises a detector comprises a plurality of photomultiplier tubes assembled in such a way that their photodiodies constitute a regular plane matrix within which each of these photodiodes, referenced k, is localized by the coordinates X.sub.k and Y.sub.k of its center.
When, under the effect of a gamma radiation pulse emitted by a radio-active isotope injected into a patient's body, a scintillation is produced by an appropriate crystal, for example a thallium-doped sodium iodide crystal, each photomultiplier tube k delivers an analog signal s.sub.k proportional to the light flux received by the corresponding photodiode.
It is then possible to localize the place of emission of this scintillation by computing the barycenter of the light spot formed on the matrix of the detector, the coordinates of the barycenter being given by: ##EQU4##
The X,Y coordinates of this scintillation are used to compute the address of a memory compartment of the detector in which there is recorded the total number n.sub.ij of scintillations detected at the point having the coordinates x.sub.i, y.sub.i, plus or minus DELTA.sub.x and DELTA.sub.y, DELTA.sub.x and DELTA.sub.y representing the sampling intervals selected along the axes x and y, these intervals being typically of the order of some millimeters. In other words, the number of events n.sub.ij is increased by 1 if: EQU x.sub.i -DELTA.sub.x /2&lt;X&lt;x.sub.i +DELTA.sub.x /2 EQU y.sub.j -DELTA.sub.y /2&lt;Y&lt;y.sub.j +DELTA.sub.y /2
The set of numbers n.sub.ij, after display, constitutes the image sought.
2. Description of the Prior Art
This image obtained by a gamma camera of the above-described type most usually has uniformity defects arising out of the variations in the number of pulses detected per unit of surface area in the field of view of the detector. These uniformity defects may have several causes. These are, firstly, defects due to statistical fluctuations caused by the random character of the emission of the scintillations, hence of the number of pulses detected per unit of surface area. It is possible, however, to minimize the effects of these defects at will by increasing the acquisition time accordingly. Secondly, they are the defects of uniformity of the detector itself. These defects are introduced, for example, by the loss of light between two contiguous photocathodes, the barycenter computation technique which introduces a relative variation in surface area, hence a variation in the number of pulses detected per unit of surface area, the fluctuations in the gain of the photomultiplier tubes which cause corresponding fluctuations in the computation of X and Y and of the energy, as well as variations in the efficiency of the scintillator crystal.
This is why, in order to obtain high quality images, it is necessary, for any gamma camera, to make a correction of uniformity. In its principle, this correction may be made as follows:
the field of view of the camera is illuminated by a uniform radiation flux by using, for example, a point source placed at very great distance from the plane of the detector,
then, a computation is made of the defects of uniformity on the image obtained, making it possible then to correct them by any method.
However, it is sometimes difficult or even impossible to place a source at a very great distance. This is either for reasons related to space requirements, for example because the room is too small, or because it would be necessary to handle sources that are excessively active. It then becomes necessary to reduce the source-detector distance. The drawback of this is that it introduces a proximity dome effect that makes the illumination of the detector non-uniform, the pixels closest to the source receiving a greater light flux.